1. Field of the Invention
The present invention relates to a semiconductor laser diode. More particularly, it relates to a semiconductor laser diode used as a light source for information processing.
2. Description of the Related Art
A conventional semiconductor laser diode has such a semiconductor structure, for example, as an n-type AlGaAs lower cladding layer 2, an active layer 3, a p-type AlGaAs upper cladding layer 4 and a p-type GaAs contact layer 6 are laminated on an n-type GaAs substrate 1 as shown in FIG. 10, wherein n-type GaAs current block layer 5 is formed on either side of the p-type AlGaAs upper cladding layer 4 which is etched out to leave a ridge portion, an n electrode 7 is formed on the bottom face of the n-type GaAs substrate 1, and a p electrode 8 is formed on the p-type GaAs contact layer 6. A semiconductor laser diode is generally used in conjunction with a drive IC. The semiconductor laser diode of the prior art shown in FIG. 10 is based on an n-type GaAs substrate 1 (cathode common laser diode), and is therefore used in conjunction with npn transistors. However, a pnp transistor is generally capable of operating at a higher speed than an npn transistor, and therefore it is preferable that the drive IC be made by using pnp transistors. For this reason, it is preferable to use a semiconductor laser diode of the so-called anode common type which has such a structure as the conductivity type of the semiconductor laser diode of FIG. 10 is reversed, namely a p-type GaAs substrate is used and a positive electrode is used as a common electrode.
However, a semiconductor laser diode of anode common type made by using an p-type GaAs substrate suffers a significant leak current and current diffuses in the n-type cladding layer which makes it impossible to inject current efficiently into an active region. This problem has been making it difficult to obtain a desired laser oscillation characteristic from a semiconductor laser diode of anode common type made by using a p-type GaAs substrate.
It can also be conceived to use a semiconductor laser diode (cathode common type) made by using an n-type GaAs substrate in conjunction with a drive IC employing pnp transistors by modifying the wiring of the semiconductor laser diode. This configuration is capable of single beam operation but is difficult to operate in multi-beam mode.